A power transmission device of such a type has heretofore been known, which includes a countershaft arranged in parallel with an input shaft of a speed change mechanism, a differential device (differential gear) including a ring gear (differential ring gear) that is arranged below the countershaft and meshes with an output gear of the countershaft, a case member accommodating the speed change mechanism and the differential device, and a differential dividing member that divides the inside of the case member into a differential chamber in which the differential device is arranged and a storage chamber for storing oil (hydraulic oil) (refer to Patent Document 1, for example). The differential dividing member of the power transmission device is constituted by a part of the case member, a rib member extending along the outer circumferential surface of the ring gear from the case member, and a hemispherical reservoir plate that covers the differential device from the side opposite to the case member and is arranged so as to be tightly attached to the inner circumferential surface of the rib member. This structure allows the differential dividing member to suppress entry of the oil from the storage chamber into the differential chamber, in the power transmission device.